Knowledge of azimuthal stress distribution of earth formation penetrated by a well borehole is useful in optimizing a variety of hydrocarbon exploration, development and production operations. As an example, knowledge of the azimuthal direction of minimum and maximum stress is useful in optimizing the orientation of horizontal well boreholes for maximum hydrocarbon production. In fractured formations, minimum formation stress is parallel to the direction of fracturing. Optimal drainage of fractured formations is obtained at an azimuthal direction parallel to the direction of fracturing.
Borehole televiewer systems can sometimes be used to optically “view” fractures intersecting a borehole. Optical borehole televiewer systems can only “see” fractures that intersect the borehole wall. Optical viewing of the borehole wall is often obliterated by wellbore fluids, by mudcake buildup on the wall of the borehole, or by other conditions that perturb optical imaging in the drilling, completion and production of a well borehole. Acoustic televiewers can penetrate some materials that render optical televiewers ineffective or useless. Acoustic borehole televiewers, which operate at relatively high frequencies, exhibit a relatively shallow radial depth of investigation into the formation. Fracturing can again be masked by intervening material between the acoustic borehole televiewer and the virgin earth formation penetrated by the borehole.
Full wave acoustic logging tools, typically comprising a plurality of axially and radially spaced transmitters and receivers, are used to determine shear wave energy distribution around a borehole. Typically at least two dipole transmitters, commonly referred to as an X-transmitter and a Y-transmitter, are oriented at 90 degrees and in a plane perpendicular to the axis of the well borehole. A plurality of receivers, each receiver typically comprising four receiver elements at the same orthogonal orientation as the transmitters, are axially spaced at predetermined distances or “stations” from the transmitters. Each receiver element in each receiver station is preferably oriented to receive a signal from either the X-transmitter or the Y-transmitter. Azimuthal shear wave energy distribution around the borehole can be obtained by combining and analyzing receiver element responses for shear wave velocity and shear wave amplitude.
In the prior art, it is typically assumed that the formation stress distribution around the borehole exhibits a minimum and a maximum that are perpendicular to each other. This assumption is, in general, not valid since it is common to encounter formations in which minimum and maximum stresses are not oriented at ninety degrees from each other. Shear wave velocity and shear wave amplitude can be used to determine stress patterns and, therefore, can be used to determine the direction of fracturing in anisotropic formations. The azimuthal variation of the shear wave parameters as a function of azimuthal stress patterns is relatively small. In order to obtain meaningful measurements, both transmitter and receiver elements must be accurately calibrated prior to a logging operation. Furthermore, calibration and “balance” between elements must be maintained while logging in a typically harsh borehole environment. Calibration and the maintaining of calibration and balance during logging are chronic problems in current full wave acoustic logging systems.